1. Field of the Invention
The present invention relates to a semiconductor integrated circuit (IC) for obtaining constant voltage characteristics or a current switching function by controlling an output current and, more particularly, to an IC using a junction field effect transistor (JFET) as a current control element.
2. Description of the Related Art
As a conventional IC for obtaining constant voltage characteristics or a current switching function by controlling an output current, a constant voltage power source IC (three-terminal voltage regulator), a switching IC, or the like is known. The constant voltage power source IC has an input terminal, an output terminal, and a common input/output terminal (in many cases, a ground terminal). A current supplied from the output terminal is adjusted by a current control element, and an output voltage generated between the output terminal and the ground terminal (common input/output terminal) is kept constant. As the current control element, an npn bipolar transistor is used. The output voltage is monitored using an error amplifier (differential amplifier). A voltage generated between the output and ground terminals and divided by a resistor is applied to one input terminal of the error amplifier, and a reference voltage output from a reference voltage source provided in the IC is applied to the other input terminal. The output from the error amplifier is fed back to the control input terminal of the current control element (the base of the npn bipolar transistor), thereby keeping the output voltage constant. When a load is connected between the output and ground terminals, if a load resistance is decreased by some reason while the IC is operated at a constant voltage, the voltage between the output and ground terminals is decreased. When the decrease in voltage is detected by the error amplifier, the output voltage of the error amplifier is increased so as to increase a base current of the npn bipolar transistor. As a result, a current supplied to the load resistor is increased, and the voltage between the output and ground terminals which is decreased is increased up to a predetermined voltage and stabilized. As the conventional constant voltage power source IC, a TA7805S, a TA78L05S, a TA79005S, and the like manufactured by TOSHIBA CORP. are commercially available.
A constant voltage power source IC in which a metal oxide semiconductor field effect transistor (MOSFET) is used in place of the npn bipolar transistor of the conventional constant voltage power source IC is known. The operation of this IC follows that of the constant voltage power source IC using the above bipolar transistor.
In the above constant voltage power source IC, when the output and ground terminals are short-circuited or almost short-circuited with each other, an overcurrent is supplied to a current control element (bipolar transistor or MOSFET), and the element is overheated and may break down. For this reason, a protection circuit is incorporated to detect the overcurrent to interrupt an output when a load current having a value larger than a predetermined value is supplied. A resistor for detecting an overcurrent is connected in series with the current control element, and an overcurrent having a value larger than a predetermined value is supplied to the resistor. At this time, the overcurrent protection circuit is operated to set the current control element to an OFF state, thereby preventing the element from being damaged.
A conventional switching IC for ON/OFF-control of an output current is arranged as follows. The switching IC has a power source input terminal, an output terminal, a common input/output terminal (ground terminal), and a control signal input terminal. A current control element (n-p-n bipolar transistor) is ON/OFF-controlled by an internal control circuit in response to a control signal supplied from the control signal input terminal. As this switching IC, a TA8057S manufactured by TOSHIBA CORP. is commercially available.
Also in the switching IC, when a load connected between the output and ground terminals is decreased by some reason and an overcurrent results, the element may be damaged. For this reason, as in a constant voltage power source IC, an overcurrent having a value larger than a predetermined value is detected by some method so as not to apply the overcurrent to the current control element. Therefore, a protection circuit consisting of, e.g., a resistor for detecting an overcurrent and an overcurrent protection circuit is generally incorporated in the switching IC.
However, the above overcurrent protection circuits are not required for essential circuit operations such as a stabilizing operation of a voltage or an ON/OFF operation of a current supply. Especially in an IC for treating a small amount of current, since another protection circuit must be added to a chip originally having a small area, the circuit becomes complicated, and the chip area is increased.